In laser isotope separation, in particular uranium enrichment on a plant scale, it is required that laser beams of hundreds of watts of power be directed over substantial distances such as thousands of meters. It is essential that the beams be long in order to insure that the useful energy within the laser beams is efficiently utilized in photoexciting or ionizing particles of a selected isotope type. At the same time, the dimensions of the channels down which the laser beams must pass are not much bigger than the beam cross-section itself. It is additionally desired that the beam be centered in the channel and not be deflected to graze or strike the channel walls. Apparatus of the sort with which such laser beams may be employed are illustrated in U.S. Pat. Nos. 3,772,519, and 3,939,354, incorporated herein by reference and commonly assigned.
Because the beams utilized in laser enrichment are typically composite beams of several colors, as well as the result of the interleaving of pulsed radiation from many pulsed laser sources, as for example illustrated in U.S. Pat. No. 3,944,947, and further because the radiation is likely to be applied through a succession of channels, it is anticipated that a number of reflecting surfaces will be required for transporting the beams from the source of generation throughout the utilization channels, as well as for aligning and redirecting the beams. Because of the power densities employed in the laser beams some radiation absorption is inevitable at the reflecting surfaces even with the most carefully prepared reflectors. As a result of such radiation absorption, the reflecting surfaces of the mirrors will increase in temperature producing a thermal expansion at the reflecting surface which, by analogy to the bimetallic strip, will result in a bending of the mirror and in particular of the reflecting surface. Such a bending produces not only an undesired shift in the beam direction over the distances of beam propagation required, but in addition produces aberations detrimental of the beam wave-front which results in defocusing and diverging effects. These may be difficult or impossible to correct.
While it has been proposed, as in U.S. Pat. No. 3,609,589 to provide a mirror of layered metallic composition wherein each layer spaced back from the reflecting surface has an increased thermal coefficient of expansion to compensate for the lower thermal heating of the reflector with distance from the reflecting surface, such layers are unsatisfactory. For a first reason, reflection by a metallic reflector is less efficient than by a layer or layers of dielectric films. In addition, the relative slowness of thermal conductivity throughout the mirror substrate prevents such a device from being effective in compensating for thermal distortions of the mirror surface on all but the most long-term basis under steady state illumination conditions. Also, the required precision to which such a mirror must be manufactured such that each layer has a precisely dimensioned thermal expansion, makes it economically impractical. Finally, absorption by a thick front face metallic reflector tends to be higher than absorption by a properly designed dielectric mirror, thus limiting total power handling capability.